Neurobiology of Pain

In human dorsal root ganglia (DRG), and trigeminal (TG) neurons, the various voltage-gated sodium channel (Nav) isoforms play critical roles in the firing of action potentials, which drive electrical impulses that encode somatosensations including, itch, and pain. The SCN11A gene encodes the tetrodotoxin (TTX)-resistant voltage-gated sodium channel Nav1.9, characterized by unique gating properties. Unlike other isoforms, the Nav1.9 channel activates and inactivates slowly and has a hyperpolarized voltage-dependence of activation and depolarized voltage-dependence of inactivation. This leads to a large window current that has been suggested to function as a regulator of the resting membrane potential of neurons. Mutations in Nav1.9 channels lead to congenital insensitivity to pain (gain-of-function) or familial episodic pain syndrome (loss-of-function) suggesting the channel is a critical mediator of pain. Despite its relevance in pain pathophysiology, most existing data relies on rodent models or heterologous expression systems, leaving the specific pharmacology and biophysical behavior of these channels in human primary neurons largely unknown. In this study, we pharmacologically isolated and characterized native Nav1.9 channel currents in human DRG and TG neurons to compare their biophysical profiles. Our findings reveal significant kinetic and voltage-dependent differences between the two populations. Specifically, Nav1.9 channels in TG neurons exhibit a right-shifted steady-state inactivation curve, a larger window current, and faster activation kinetics compared to those in DRG neurons. In addition, conditions that simulate inflammatory states in-vivo greatly potentiates the Nav1.9 currents consistent with similar observations in rodent models. By detailing these distinct biophysical properties, this research offers crucial insights into Nav1.9 channel function relevant for drug discovery efforts aimed at developing analgesics for both acute and chronic pain.

Understanding the complex network that regulates pain is fundamental to develop strategies to combat its growing prevalence and increase useful therapeutics. Although extensive literature identifies the importance of cannabinoid receptors and endocannabinoids in controlling pain, their efficacy and loci of action remain debated. To directly test the actions of peripherally restricted cannabinoids and elucidate the minimal circuitry capable of producing cannabinoid-mediated analgesia, we utilized a novel genetic approach that allows for cell-specific reactivation of cannabinoid receptor 1 (CB1R) selectively in peripheral sensory neurons using newly developed CB1R floxed-stop-floxed mice (CB1RLOXTB) crossed with Nav1.8-cre mice (Nav1.8+/-:CB1RLOXTB). Ex vivo and in vivo experiments confirmed successful knockout and reactivation of CB1R. Wildtype littermate controls, but neither Nav1.8+/-:CB1RLOXTB nor CB1RLOXTB animals, exhibited robust analgesia after systemic WIN55,212-2 (WIN) treatment in the tail flick assay. Furthermore, the presence of CB1R on Nav1.8 neurons was not associated with either a difference in the development of inflammatory pain or the response to WIN. However, after neuropathic injury, CB1RLOXTB animals displayed an earlier onset of both mechanical and thermal hypersensitivity than their Nav1.8+/-:CB1RLOXTB or wildtype counterparts, suggesting a dual role for CB1R in inflammatory and neuropathic pain. These studies represent an important approach to further improve our mechanistic understanding of cannabinoid modulation of pain in the nervous system and begins to settle long-standing controversies in cannabinoid literature. Table of ContentsPeripherally restricted cannabinoids show strong preclinical analgesic efficacy but have not translated clinically. Using a genetic model restricting CB1R to Nav1.8-expressing sensory neurons, we show peripheral neuronal endocannabinoid signaling is required for chronic, but not acute pain modulation. This dissociation suggests clinical failures may reflect testing peripheral cannabinoids in acute rather than chronic pain paradigms, informing future translational strategies.

Nerve growth factor (NGF) is a key mediator of pain through activation of the high-affinity tropomyosin receptor kinase A (TrkA) and the low-affinity neurotrophin receptor (p75NTR). Although neuronal TrkA signaling is well established, the contribution of non-neuronal cells to NGF- dependent pain remains unclear. Here, we show that NGF and its precursor proNGF engage distinct cellular mechanisms. Intraplantar NGF induced acute nociception, heat hyperalgesia, mechanical allodynia, and cold hypersensitivity, whereas cleavage-resistant proNGF selectively evoked mechanical allodynia and cold hypersensitivity. Pharmacological and cell-specific genetic approaches demonstrated that acute nociception and heat hyperalgesia require neuronal TrkA, whereas mechanical and cold hypersensitivity depend on p75NTR activation in Schwann cells. In Schwann cells, NGF and proNGF induced p75NTR-dependent calcium release, followed by TRPA1 activation, mitochondrial ROS production, and NOX1-dependent oxidative amplification. Inhibition of ROS or TRPA1, or Schwann cell-specific Trpa1 deletion, markedly reduced mechanical allodynia and cold hypersensitivity without affecting acute nociception or heat hyperalgesia. These findings identify a Schwann cell p75NTR-ROS-TRPA1 pathway sustaining persistent pain and highlight non-neuronal p75NTR signaling as a potential therapeutic target.

Spinal cord stimulation (SCS) is an established therapy for neuropathic pain, typically delivered at either low (60 Hz) or high (1 kHz) frequencies, with analgesic effects largely dependent on active stimulation. Here, we investigated whether combined-frequency SCS produces sustained analgesia beyond stimulation periods and explored the underlying mechanisms. Using a spared nerve injury (SNI) model in both rats and mice, we applied dual-frequency SCS (60 Hz + 1 kHz). This paradigm produced robust reversal of mechanical allodynia during stimulation and, notably, a progressive and long-lasting analgesic effect that persisted for days to weeks after stimulation cessation. RNA sequencing revealed pronounced immune-related transcriptional changes in the spinal cord, including upregulation of innate immune, pro-resolution, and neutrophil-associated pathways. Functional studies demonstrated that neutrophil depletion attenuated SCS-induced analgesia, whereas intrathecal S100A8 treatment mimicked therapeutic effects via CD69/SOCS3 signaling. These findings identify dual-frequency SCS as a promising strategy to prolong analgesia and highlight a critical role for neuroimmune modulation in sustained pain relief. HighlightsO_LICombined-frequency, not single-frequency SCS, sustains analgesia during washout C_LIO_LICombination SCS induces robust immune activation in spinal cord and DRG C_LIO_LICombination SCS increases spinal perfusion and promotes neutrophil recruitment C_LIO_LINeutrophil signaling contributes to sustained SCS analgesia C_LI

Urinary tract infections (UTIs) are a major risk factor for interstitial cystitis/bladder pain syndrome (IC/BPS), yet the mechanisms driving chronic pelvic pain and associated symptoms remain poorly understood. Here, we investigated the role of microglia and Toll-like receptor 4 (TLR4) in a mouse model of post-UTI chronic pelvic pain (PUPP). Infection with E. coli induced persistent pelvic allodynia that was significantly attenuated by microglial depletion (PLX5622) or inhibition (minocycline), indicating a key role for microglia in pain maintenance. In contrast, microglial depletion did not improve urinary dysfunction or anxiety- and depression-like behaviors. Prefrontal cortex microglia of PUPP mice exhibited reduced microglial branching complexity and a less ramified phenotype, indicative of an activated microglial state. Transcriptomic profiling of brain CD11b+ cells revealed a reactive microglial signature enriched for chemokines, NFKB-related genes, and immediate early response genes, alongside pathways involved in immune regulation and leukocyte recruitment. Both general and microglia-specific TLR4 deletion reduced pelvic allodynia and reduced microglial morphological features of activation. Consistent with this, pharmacological TLR4 inhibition in vitro suppressed LPS-induced NFKB activation, cytokine secretion, and CD68 expression. Together, these findings identify microglial TLR4 as a critical mediator of post-UTI chronic pelvic pain.

Cannabidiol (CBD) is a non-psychoactive component of cannabis that has been studied as a potential therapy for chronic pain. CBD attenuates behavioral hypersensitivities in models of neuropathic pain, and promotes production of bioactive lipids (e.g., anandamide), altering lipid signaling. However, a lack of understanding of the mechanisms underlying the therapeutic effects of CBD has hindered development and application of CBD to mechanism-based therapies for pain in people. We asked whether the analgesics effects of CBD were dependent upon the enzyme NAPE-PLD. We used a mouse model of chemotherapy-induced peripheral neuropathy (CIPN) to evaluate the acute and chronic antinociceptive effects of CBD and investigate its mechanisms. Pharmacological specificity was tested with antagonists targeting CB1, CB2, PPAR{gamma}, and PPAR receptors. Mechanisms were further examined using NAPE-PLD and GPR55 knockout mice. We also assessed repeated CBD dosing during both the development and maintenance of paclitaxel-induced CIPN in wild-type, GPR55 KO, and NAPE-PLD KO mice. CBD suppressed paclitaxel-induced behavioral hypersensitivities; these effects were attenuated by a PPAR and PPAR{gamma} antagonists, but not CB1 or CB2 antagonists. CBD reduced both the development and maintenance of neuropathic nociception in a model CIPN in wild-type mice, but these effects were absent in NAPE-PLD KO mice. By contrast, anti-allodynic efficacy of CBD was fully preserved in GPR55 KO mice. Pharmacological blockade of the PPAR receptor and genetic deletion of NAPE-PLD abolished the antinociceptive effects of CBD in a model of CIPN, suggesting a pivotal role for NAPE-PLD and PPAR receptors in CBD-mediated analgesia in chemotherapy-induced neuropathic pain.

Patients with bowel disease can develop referred somatic pain at a later time with unknown molecular mechanisms. Using experimental mice with colitis induced by intracolonic installation of 2,4,6-Trinitrobenzenesulfonic acid (TNBS), we find an increase in the percentage of hind paw primary afferent neurons expressing Piezo2 or calcitonin gene-related peptide (CGRP), which are attenuated by TrkB.T1 knockout (KO). Concomitantly, TrkB.T1 KO also attenuates colitis-induced hind paw mechanical hypersensitivity and pain. Next, we find that TrkB.T1 is expressed in spinal cord astrocytes and its expression level is increased by colitis. TrkB.T1 KO reduces colitis-induced upregulation of Tumor necrosis factor-alpha (TNF-) mRNA but not upregulation of interleukin (IL)-6 mRNA in the spinal cord. Using calcium (Ca2+) imaging and ex vivo approaches, we find that TNF- elicits Ca2+ transients in capsaicin-sensitive as well as capsaicin-insensitive DRG neurons, and increases Piezo2 and CGRP expression in DRG neurons via distinct signaling pathways. Notably, TNF-or colitis-induced Piezo2 upregulation in L4 DRG neurons is mediated by or associated with the PI3K/Akt pathway that does not participate in CGRP upregulation. In contrast, CGRP upregulation in hind paw primary afferent neurons is associated with an upregulation of phosphorylated cAMP response element binding protein (p-CREB). Finally, we find that TrkB.T1 KO does not change the expression level of transient receptor potential cation channel subfamily V member 1 (TrpV1) in L4 DRG in colitis, explaining the ineffectiveness of TrkB.T1 KO on colitis-induced hind paw thermal hyperalgesia. These results suggest complex and distinct molecular pathways in colitis-induced somatic pain modalities and provide information for specific pain modality management.

Chronic pain in later life may be accompanied by alterations in brain structure and cognition, but whether pain extent and central sensitization symptoms identify distinct brain-behavior patterns remains unclear. We examined associations of pain extent and central sensitization symptoms, assessed using the 9-item Central Sensitization Inventory (CSI-9), with regional gray matter volume and cognitive function in community-dwelling older adults. This cross-sectional study included 272 participants with chronic pain from the Bunkyo Health Study. Participants were classified as having single-site or multisite pain and by CSI-9 score as having lower or higher scores, with 12 or higher defining the higher group. Regional gray matter volume was quantified using 0.3-Tesla magnetic resonance imaging, and cognition was assessed using the Trail Making Test Part B (TMT-B), processing speed, and global and domain-specific measures. Pain extent and CSI-9 group interacted for TMT-B performance, with the longest completion time in participants with single-site pain and a higher CSI-9 score. No other cognitive outcome remained significant after correction for multiple testing. In categorical analyses, the higher CSI-9 group had smaller volumes in the right middle frontal gyrus, bilateral anterior cingulate cortex, right insula, right hippocampus, and bilateral amygdala, whereas pain extent and the interaction were not associated with regional volume. In a contextual comparison, only the single-site/higher CSI-9 group showed slower TMT-B performance than participants with no current pain. Pain extent and central sensitization symptoms may represent partly distinct dimensions of chronic pain, although the small single-site/higher CSI-9 group and attenuation in several sensitivity analyses warrant caution. Significance StatementChronic pain is often described by where it hurts, but location alone may miss important differences between patients. In older adults, pain extent and symptoms measured by the 9-item Central Sensitization Inventory captured partly different aspects of chronic pain. Participants with pain at one site and a higher CSI-9 score performed most slowly on a task requiring attention and mental flexibility, whereas differences in regional brain structure were related mainly to CSI-9 score rather than pain extent. These findings support a multidimensional approach to chronic pain and may inform future research on cognitive vulnerability and brain health across pain conditions. Graphical Abstract Text O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=120 SRC="FIGDIR/small/742487v1_ufig1.gif" ALT="Figure 1"> View larger version (47K): org.highwire.dtl.DTLVardef@17eacb7org.highwire.dtl.DTLVardef@17d535borg.highwire.dtl.DTLVardef@ebb79forg.highwire.dtl.DTLVardef@16464b7_HPS_FORMAT_FIGEXP M_FIG C_FIG Among older adults with chronic pain, pain extent and CSI-9 score captured different aspects of vulnerability. Slower performance on a task requiring attention and cognitive flexibility was concentrated in those with single-site pain and higher CSI-9 scores, whereas regional brain-volume differences tracked CSI-9 category more broadly.

BackgroundThis study aimed to determine if a rat osteotomy model elicits a measurable systemic response and to utilize this profile to evaluate the mechanism of action of preemptive analgesics with translational relevance to veterinary perioperative pain management. MethodsTwenty-five male rats were randomized into five groups: Sham, Surgery Control, Robenacoxib (2 mg/kg S.C.), Amantadine (30 mg/kg P.O.), and Combination. A femoral osteotomy was performed following ARRIVE 2.0 guidelines for refinement of surgical models. Serum levels of IL-6, PGE2, and cortisol were quantified via ELISA at baseline, 1-, 3-, and 6- hours post-surgery. Postoperative pain was assessed using the Rat Grimace Scale (RGS). ResultsThe osteotomy model did not induce a significant systemic inflammatory or stress response. Serum IL-6 and cortisol levels showed no significant changes over time (IL-6: p=0.219; Cortisol: p=0.187) or between groups. While PGE2 showed a temporal increase (F=6.52, p=0.001), it was unaffected by drug treatments. In stark contrast, the model successfully produced significant pain-related behaviors in the Control group (p=0.007), which were effectively reduced by both Robenacoxib (p=0.014) and amantadine (p=0.019) monotherapies. The combination group also showed significant pain reduction compared to Control at T6 (p=0.002), with an additive effect relative to monotherapies. Correlation analysis confirmed a dissociation between systemic biomarker levels and pain scores. ConclusionThe efficacy of Robenacoxib (a COX-2 inhibitor approved for veterinary use) and amantadine in the absence of altered systemic biomarkers suggests their analgesic actions are mediated predominantly through local or neurogenic pathways, with direct implications for optimizing perioperative analgesia protocols in companion animal orthopedic surgery.

Chronic pain is highly heterogeneous, with individuals varying substantially in symptoms. pain severity, disability, affective distress, cognitive functioning, and trauma-related symptoms. This study examined whether working memory, post-traumatic stress symptoms (PTSS), trauma exposure, and pain modulation explain distinct or shared dimensions of chronic pain variability. Individuals with chronic pain completed clinical, cognitive, trauma-related, and behavioural pain modulation measures, as well as resting-state functional magnetic resonance imaging. Multivariate regressions were used to determine whether working memory, PTSS, trauma exposure, and pain modulation independently predicted chronic pain outcomes. Principal component analysis was used to identify latent dimensions of chronic pain, and mediation analyses tested whether behavioural pain modulation explained relationships between dlPFC to vlPAG resting-state functional connectivity and clinical pain outcomes. PTSS independently predicted affective outcomes, including depression, state anxiety, and trait anxiety, whereas working memory independently predicted pain severity and pain interference. Trauma exposure was associated with greater PTSS and poorer working memory, but did not independently predict core pain outcomes after accounting for these more proximal factors. Principal component analysis identified partially distinct affective and sensory-disability dimensions, while trauma exposure loaded primarily on a separate component characterized by greater PTSS and poorer working memory. Behavioural pain modulation showed broader relationships across symptom dimensions and was associated with dlPFC to vlPAG connectivity. Exploratory mediation analyses demonstrated that pain modulation mediated relationships between dlPFC to vlPAG connectivity and both pain severity and affective distress. These findings support an integrated model where PTSS and working memory are more proximal predictors of affect and severity respectively, and trauma exposure represents a more distal vulnerability factor that predicts both. Thus, pain modulation represents a shared mechanism linking cortico-brainstem connectivity to chronic pain intensity and affect. These variables need further testing for phenotyping people with chronic pain based on their specific clinical needs.

Nociceptive pain is the most common pain condition, and moderate-to-severe nociceptive pain substantially impacts daily functioning, constituting a significant public health burden. Despite this, most studies investigating the neural mechanisms underlying somatosensory processing and inhibition have focused on other pain conditions (e.g., neuropathic, nociplastic, or mixed pain). Thus, the extent to which neural aberrancies detected in these other populations extend to or differentiate from nociceptive pain conditions remains largely unknown. In this study, 29 individuals with moderate-to-severe nociceptive pain (MSNP) and 47 pain-free (PF) controls underwent magnetoencephalography (MEG) alongside a paired-pulse somatosensory stimulation paradigm to examine somatosensory cortical processing and functional inhibition. Pain status and intensity were determined using validated pain questionnaires, painDETECT and PROMIS-29, respectively. MEG oscillatory responses were source localized via a beamformer to the primary somatosensory cortex (S1) and time series data were extracted from the peak voxel to quantify the dynamics of somatosensory gating (SG; index of cortical inhibitory processing), oscillatory response power, and spontaneous power. We found that adults with MSNP exhibit aberrant theta SG in contralateral S1 compared to PF controls, reflecting reduced functional inhibition of innocuous stimulus processing in this region. Additionally, individuals with MSNP demonstrated exaggerated gamma responses but blunted alpha responses in contralateral S1 to innocuous stimulation. Finally, individuals with MSNP were characterized by weaker spontaneous alpha in contralateral S1 that scaled with self-reported pain intensity. Together, these findings suggest that experiencing MSNP is associated with disrupted somatosensory and cortical inhibitory processing.

Effective pain relief is a central goal of medical care, yet objective biomarkers of pain relief are lacking. Using task-based functional MRI and a capsaicin-induced tonic heat pain model, we experimentally elicited both pain exacerbation and pain relief within the same individuals. Existing brain-based signatures, including the Neurologic Pain Signature (NPS), reliably detected pain increases, but failed to capture pain relief. We therefore developed the PAin and RElief Signature (PARES), a multivariate brain signature trained to predict bidirectional changes in pain perception in n = 61 healthy controls. PARES robustly predicted both pain increases and relief and generalized to an independent cohort of people with chronic back pain (n = 58), who underwent the same experimental procedures. Together these findings establish a neural signature of pain relief and provide a potential biomarker for treatment stratification and analgesic development.

Identifying objective translational biomarkers of spinal nociceptive processing is important to accelerate analgesic development. The primary negative component (N1) of spinal somatosensory evoked potentials (SEPs) has been proposed as such a biomarker. However, the cellular substrates of the N1 potential (evoked by innocuous electrical stimulation) and their relevance to nociceptive processing have not been directly demonstrated. Here, we employed a 64-channel multielectrode recording approach in the dorsal horn of anaesthetised Wistar rats to functionally characterise the neuronal populations activated during the generation of spinal SEPs and determine how their activity is modulated by tapentadol. Single units were classified based on their responses to mechanical stimulation of the hindpaw, and their electrically evoked responses to sciatic nerve stimulation. Of 59 well-isolated units, 47 (80%) were classified as wide dynamic range (WDR) neurons and 12 (20%) as low-threshold mechanoreceptive (LTMR) neurons, spatially distributed across spinal laminae III-V. Tapentadol (10 mg/kg, intraperitoneal (i.p)) selectively attenuated the mechanically- and electrically-evoked activity of WDR neurons without affecting LTMR responses. This WDR-inhibition was largely reversed by naloxone (0.25 mg/kg, i.p) but not by atipamezole (1 mg/kg, i.p), identifying a predominant opioid receptor-mediated mechanism of inhibition in the naive state. The magnitude of WDR inhibition by tapentadol correlated with the degree of reduction of the N1 amplitude. These findings establish activity in WDR neurons as a core component of the N1 potential, supporting the use of spinal SEPs as a translational biomarker of analgesic target engagement within the dorsal horn. SummaryMultielectrode recordings identify inhibition of WDR neurons as the mechanism by which tapentadol modulates spinal SEPs, supporting use as a biomarker of spinal nociceptive processing.

BackgroundThe Delta-opioid receptor (DOR) has gained attention as a promising target for the treatment of migraine and headache disorders. This is largely attributed to its unique pharmacological profile, which suggests that DOR-targeting treatment offers effective therapeutic benefit with a lower risk of medication overuse headache (MOH), reduced abuse liability, and minimal potential for physical dependence. These advantages have driven the development of a novel DOR agonist PN6047 (3-[[4-(dimethylcarbamoyl) phenyl]-[1-(thiazol-5-ylmethyl)-4-piperidylidene] methyl]benzamide), which has completed Phase I clinical trial and showed a favorable safety and tolerability profile. Although PN6047 has shown promising effects in neuropathic pain models, its efficacy in preclinical models of headache-associated pain remains to be evaluated. Here, we investigated the effects of PN6047 in models of migraine-associated pain and aura as well as post-traumatic headache (PTH) and MOH. MethodsC57BL6/J mice were used to examine the effects of PN6047 in the following migraine models: chronic intermittent nitroglycerin (NTG)-induced migraine-associated pain, PTH, KCl-induced cortical spreading depression (CSD), and optogenetic evoked CSD in a freely behaving transgenic mice expressing ChR2-eYFP. In addition, we tested whether chronic PN6047 induced MOH and whether it could prevent the development of MOH induced by sumatriptan. ResultsA single injection of PN6047 blocked chronic cephalic allodynia established by chronic intermittent NTG and PTH. Moreover, chronic PN6047 treatment prevented the development of MOH induced by sumatriptan, without causing MOH itself. In addition, PN6047 significantly reduced the number of CSD events in the KCl-induced CSD model, and delayed CSD onset triggered in freely behaving mice along with subsequent CSD-evoked allodynia. ConclusionPN6047, a novel DOR agonist, strikingly blocks headache-associated mechanism and symptoms in preclinical models of chronic migraine, migraine aura, PTH, and MOH. Importantly, prolonged PN6047 treatment did not induce MOH or analgesic tolerance. Together, these data demonstrate that despite the distinct mechanisms underlying migraine and headache disorder, PN6047 exhibits robust efficacy without inducing MOH, and displays a favorable safety and tolerability profile.

Paclitaxel-induced peripheral neuropathy (PIPN) is the most common dose-limiting side effect of paclitaxel chemotherapy, yet the relationship between structural damage to peripheral nerve terminals and functional impairment of nociceptors has not been directly examined. Here we combined ex vivo skin-nerve electrophysiology with three-dimensional (3D) imaging of fDISCO-cleared glabrous skin to characterize both morphological and functional changes in peripheral sensory terminals following paclitaxel treatment in rats. Five weeks after paclitaxel administration, skin-nerve recordings revealed a marked increase in the proportion of C- and A{delta}-fibers exhibiting spontaneous discharge (approximately 2.4- and 2.6-fold increases, respectively) compared to vehicle-treated controls. Paclitaxel also selectively reduced the mechanical activation threshold of C-fibers without affecting A{delta} fibers. Three-dimensional reconstruction of PGP9.5-immunolabeled nerve terminals showed that sensory fibers in glabrous skin form a vertically oriented, tree-like architecture. Paclitaxel treatment severely reduced both the terminal branch length and the density of free nerve endings in the epidermis. Strikingly, co-administration of the Kv7 channel activator retigabine prevented both the electrophysiological and morphological alterations induced by paclitaxel. These findings provide direct evidence that peripheral nerve terminal degeneration and hyperexcitability co-occur in PIPN and that Kv7 channel activation can protect against both structural and functional damage. SignificanceThis study provides the first direct evidence linking intraepidermal sensory terminal degeneration with peripheral sensory fiber hyperexcitability in paclitaxel-induced peripheral neuropathy by combining skin-nerve electrophysiology with three-dimensional imaging of sensory terminals. Paclitaxel induces distal degeneration of intraepidermal sensory terminals, increases spontaneous discharge, and lowers the mechanical activation threshold of C-fibers, demonstrating that structural degeneration and functional abnormalities occur concurrently in the peripheral terminal. Retigabine prevents both structural and functional alterations, supporting peripheral sensory terminals as a therapeutic target for preventing chemotherapy-induced neuropathy.

Background: Accurately identifying patients who will require opioids after third molar extraction could improve pain management while supporting opioid stewardship. This study evaluated surgeon accuracy in predicting supplemental opioid use following treatment with ibuprofen and acetaminophen. Methods: Patients (N=85) undergoing third molar extraction were treated with a standardized analgesic regimen of ibuprofen+acetaminophen, with supplemental opioid if needed. Four surgeons independently reviewed preoperative radiographs, assessed surgical difficulty using the Pederson scale, and rated the likelihood of supplemental opioid use on a 5-point Likert scale. Inter-rater reliability was assessed using intraclass correlation coefficients (ICC). The relationship between surgeon ratings and postoperative opioid use was evaluated using logistic regression and receiver operating characteristic (ROC) analysis. Results: Seventeen patients used supplemental opioid analgesics. Inter-rater reliability among surgeons was moderate (ICC3=0.606, 95%CI: 0.505-0.700), while reliability of the average rating across surgeons was good (ICC3k = 0.860, 95% CI: 0.804-0.903). Median surgeon rating was not associated with postoperative opioid use (OR: 0.800, 95% CI: 0.414-1.51, p=0.496) and demonstrated poor discrimination (AUC: 0.551, 95% CI: 0.392-0.710). Surgeon ratings were positively associated with Pederson score (beta=0.073, 95%CI: 0.050-0.096; p<0.001). Conclusions: Surgeons demonstrated moderate agreement, but these assessments did not accurately identify patients who ultimately required supplemental opioids. Surgeon judgments appeared to be influenced by anticipated surgical difficulty. Practical Implications: Clinicians should follow current recommendations against routine "just-in-case" opioid prescribing after third molar extraction. Future studies should focus on identifying clinical and biological predictors of inadequate analgesic response to NSAIDs to support individualized pain management strategies.

Background. Pain influences motor function and has been proposed to reduce corticospinal and intracortical excitability. At the same time, performance can be maintained during pain, at the cost of increased perceived effort, a centrally generated signal reflecting resource engagement. Here, we tested whether contralateral thermal heat pain-related changes in corticospinal and intracortical excitability contribute to increased effort perception. Methods. In this preregistered transcranial magnetic stimulation (TMS) study, twenty-one healthy participants received single and paired pulse TMS at rest and during submaximal isometric right wrist flexions performed at 20% maximal peak force. Trials were conducted under a control condition or during contralateral thermal stimulation (painful or non-painful warm) applied to the left forearm. After each contraction, participants rated the intensity of their perceived effort. Corticospinal and intracortical excitability of the right wrist flexor was assessed at rest and during submaximal contractions. Results. Contralateral heat pain significantly increased perceived effort compared with the control and warm conditions. Contralateral heat pain did not reduce corticospinal or intracortical excitability. Conversely, contralateral heat pain increased corticospinal excitability, reflected primarily in decreased cortical silent period duration. Perceived effort was associated with the subjective experience of pain rather than with TMS-derived variables. Conclusions. These findings suggest that increased effort during contralateral heat pain cannot be attributed to inhibition of the primary motor cortex or the corticospinal pathway. The higher perceived effort in the presence of contralateral heat pain likely reflects the cognitive cost of pain rather than alterations in the transmission of the motor command.

Chemotherapy-induced peripheral neuropathy (CIPN) is a common and painful side effect of paclitaxel (PTX) treatment. The most common measures of painful neuropathy focus on evoked mechanical hypersensitivity, but clinically relevant ongoing pain remains understudied in preclinical models. Automated machine learning methods for pose estimation and behavioral classification have been proposed to capture non-evoked pain-like behaviors, though these approaches have primarily been applied to unilateral injury models such as spared nerve injury or unilateral inflammatory compound injection. Here, we evaluated the extent to which paclitaxel-induced CIPN affects the posture and spontaneous behavior of freely moving mice using a commercially available automated recording system (BlackBox). We found that paclitaxel-treated mice develop a broad and reproducible behavioral and postural phenotype relative to vehicle-treated controls, characterized by reduced front paw luminance and print size, increased front paw lifting, and altered body measurements consistent with a guarded posture. This phenotype was replicated across two independent cohorts and was detectable at both day 2 and day 6 following the final paclitaxel injection. To identify behavioral features specific to CIPN, we administered gabapentin, an analgesic often used to treat neuropathic pain in patients, to determine whether paclitaxel-induced behavioral changes could be attenuated. Gabapentin reduced several behavioral features in both paclitaxel-treated and vehicle-treated animals, suggesting that its effects on posture and gait are not specific pain in CIPN. These findings demonstrate that automated behavioral recording captures a robust paclitaxel-induced postural phenotype but question whether captured behaviors are indicative of ongoing pain as alleviated by gabapentin.

Endometriosis is a prevalent condition characterized by chronic pelvic pain that is frequently refractory to treatment. While the mechanisms underlying this pain remain poorly defined, clinical evidence often indicates that lesion innervation, but not disease stage (e.g. number and depth of lesions), correlate with pelvic pain severity. However, characterization of lesion-innervating neurons is incomplete, revealing an opportunity to identify novel, disease-modifying therapeutics. Here, we coupled functional analyses of lesion-innervating neurons in a mouse model with concurrent identification and characterization of lesion-innervating neurons from pain-phenotyped endometriosis patients. Following the confirmation of abdominal-directed pain-like behaviors in the mouse model, electrophysiological analysis revealed that lesion-innervating dorsal root ganglion (DRG) neurons are hyperexcitable compared to matched controls. These neurons are predominantly small-diameter and bind Isolectin B4, an established marker of the GDNF Family Ligand receptor, Ret. GDNF is concentrated within the stromal layer of both mouse and human lesions, adjacent to axons expressing the GDNF co-receptor, GFR1. Critically, clinical pain correlates with lesion GDNF level, axonal density, and neuronal GFR1 levels. These data provide evidence that endometrial lesions may recruit the Ret-positive subpopulation of nociceptors where they become sensitized and increase patient pain.

Background: Chronic pain affects approximately 20% of children and youth worldwide and is associated with mental and physical health impacts. Canada-specific data on the prevalence of chronic pain in children and youth are limited, highlighting the need for current high-quality population-based estimates Aims: The aim of this study is to provide national estimates of self-reported chronic pain among Canadian children and youth by pain type (headache stomach ache, backache), sex (female, male), age group (5-11, 12-17 years) and province or territory. Methods: Publicly available data were used from the 2019 Canadian Health Survey on Children and Youth (CHSCY), a population-based survey conducted by Statistics Canada using a nationally representative sample of Canadian children and youth Results: Overall, headaches were the most commonly reported pain type (15.4%), followed by stomach aches (12.5%), and backaches (11.1%). Prevalence was consistently higher among females than males and among youth than children, with youth girls reporting the highest prevalence across all pain types. Prevalence also varied geographically, with some of the highest estimates observed in the Atlantic Provinces. Conclusions: Chronic pain affects substantial proportions of Canadian children and youth with disparities observed by pain type, sex, age, and geography. These findings under score pediatric chronic pain as an important public health issue and highlight the need for equity-oriented approaches that address the needs of populations experiencing the greatest burden.